Optical disk media, such as a DVD-RAM and a DVD-RW, that use a rewritable type phase change optical recording material, have been available widely for general use. Usually, the optimization of the power and pulse conditions of a recording laser beam, called “trial writing”, is important in the recording of information in these high-density optical disks.
Usually, a jitter, that is a standard deviation of the shift between a data edge and a clock edge, is used to estimate the quality of a signal recorded in an optical disk. A measuring apparatus, such as an exclusive jitter analyzer, is typically necessary to measure jitter. Because an expensive jitter analyzer cannot be incorporated in an optical disk apparatus as a practical matter, a signal estimation index was required instead of this expensive jitter analyzer.
As an example of a conventional trial writing technique, the technique of estimating jitter equivalently and optimizing the recording power using this value by generating a logical pulse, that is called an error pulse, and counting an error pulse count, instead of measuring the jitter directly, when the phase difference between a data edge and a clock edge exceeds a predetermined value, is disclosed in JP-A No. 320777/1998.
A trial writing technique for a 4.7 GB DVD-RAM that uses an adaptive recording strategy of a table reference type conforming to the front and rear space length and mark length is disclosed in the International Laid-Open Patent Publication No. WO01/011614. In accordance with this technique, classification processing is applied by making an error pulse correspond to a table of the recording strategy, and a pulse condition of a recording laser beam is optimized so that the error pulse values of table items may be minimized. Practical classification processing requires two 4×4 tables. However, because a logical pulse, called an error pulse, is used to estimate a signal, the classification processing can be realized simply by a logic LSI.
[Patent document 1] JP-A No. 320777/1998
[Patent document 2] WO01/011614
As described previously, at the present time, when the use of a DVD has become widespread, a fast recording/reproduction speed is becoming a most important technical development problem. Because the noise effect increases when a signal is reproduced at high speed, a PRML (Partial Response Maximum Likelihood) method that improves the S/N (Signal-to-Noise) ratio effectively is becoming indispensable as a binary system of a reproducing signal, instead of a conventional direct slice method. The PRML method requires a process of digitalizing the reproducing signal at every clock using an A/D (Analog-to-Digital) converter. Necessarily, a PLL (Phase-Locked Loop) circuit also changes from a conventional analog system to a digital system. In the PLL circuit of the digital system, a VCO (Voltage Controlled Oscillator) circuit is operated so that the level of the reproducing signal at an edge point may approach zero, instead of directly comparing the phase difference between a data edge and a clock edge. Because a conventional error pulse generation circuit adapts to a phase error detection circuit of the PLL of an analog system, it could not be adapted to a PLL of the digital system.
When a PRML method and a digital PLL are adopted in this manner, a conventional error pulse generation circuit will not function. When a new signal estimation index is introduced instead of an error pulse, the configuration of a conventionally developed logic LSI, and a control program resource will not be able to be used effectively.
Moreover, the PRML method executes binarization so that apparently the most accurate data row may be selected sequentially by comparing not only the position of the edge of a reproducing signal, but also the whole signal, with a target signal. In this case, about two to five numeric rows, called a PR class, are used to generate the target signal. The PR class allows an impulse response of the reproducing signal to approximate the numeric rows. A PR(3,4,4,3)ML method is well known as a PR class for a DVD. However, because the PR class approximates the impulse response to the end, for example, a write pulse or power is determined using a PR(3,4,4,3)ML decoder, so that signal estimation indexes peculiar to the PRML method, such as a bit error rate, a SAM (Sequenced Amplitude Margin), and an MLSE (Maximum Likelihood Sequenced Error) may be used so as to optimize these. In this case, because the write pulse or recording power differs from the waveform equalizing condition defined in the DVD standard, jitter increases when a signal is reproduced by a conventional apparatus, and the reproduction compatibility performance between optical disk apparatuses will be deteriorated.
Moreover, there are a DVD-RAM, a DVD-R, a DVD-RW, a DVD+R, and a DVD-RW as recording type DVD media. The use of one drive to enable recording/reproduction corresponding to these disks is anticipated. The numerical apertures of a reference drive head defined in these standards are 0.60 for the DVD-RAM, DVD-R, and DVD-RW, and 0.65 for the DVD+R and DVD+RW. A single optical head mounted on the drive must select either of the numerical apertures in order to correspond to all of these disks. Usually, it is natural to adopt the larger numerical aperture 0.65, and to record and reproduce a signal having a small spot size when considered from the point of view of signal quality or the efficiency of recording power in the case of high-speed recording. In this case, when the trial writing of a DVD-RAM is executed, the write pulse condition must be optimized using an optical head having an NA of 0.65 so that the signal is reproduced by an optical disk apparatus having an the of NA 0.60, and the jitter may be improved.